"Northmen" was a common term for the Vikings, famously used in the prayer "From the fury of the Northmen deliver us, O Lord!", doubtfully attributed to monks of the English monasteries plundered by Viking raids in the 8th and 9th centuries. The word "Norman" is a derivation.

The Northmen were also known as Ascomannii by the Germans (perhaps due to their mythological ancestor Ask), Lochlanach by the Irish and Dene (Daner) by the Anglo-Saxons. The Slavs, the Arabs and the Byzantines knew them as the Rus' or Rhos (probably from various uses of roþs-, i.e. "related to rowing", hence Russia). The Slavs and the Byzantines also called them Varangians (Væringjar, meaning "sworn men"), and the Scandinavian bodyguards of the Byzantine emperors were known as the Varangian Guard.

J. R. R. Tolkien adopted the term in his fiction; his "Northmen" were Men that lived in the north of Rhovanion in Middle-earth, and were friendly to Gondor.

The Northmen who dwellt in Greenwood the Great and other parts of Rhovanion were friendly to the Dúnedain and were after the Dúnedain the most noble Men on Arda, counted as Middle Men by the Dúnedain, and believed to have been descended from the same group of Men as the Edain, the Atanatári. The only difference was that they didn't cross the Ered Luin into Beleriand and therefore didn't go to Númenor. The result of them not participating in the War against Morgoth was their considerably shorter lifespan if compared to the lifespan of the Dúnedain, whose lifespan was enhanced by the Valar after the War of Wrath. They were important allies of Gondor and served as a buffer against the Easterlings, and in the Army of Gondor. For a time many of them even became subjects of Gondor, as the realm extended beyond the river Anduin.

East of Greenwood the Great was the kingdom of Rhovanion, and this became the most important nation of the Northmen. In the fourteenth century of the Third Age, King Rómendacil II of Gondor sent his son Valacar as an ambassador to Vidugavia, the so-called "King of Rhovanion". Valacar loved Rhovanion and the daughter of the King, Vidumavi. He married her. and she bore him a son whom she called Vinitharya in her mother tongue. In Gondor Vinitharya became known as King Eldacar and caused a civil war, the Kin-strife because of his mixed Dúnadan/Northmen blood.

Shortly after the death of Vidugavia a small group of Northmen moved to the vales of the Anduin between the Gladden Fields and the Mirkwood, probably to flee from the Wainriders, who had enslaved Rhovanion. After the evil Kingdom of Angmar was defeated by Gondor and the remains of Arnor in 1977 T.A., these Northmen moved north and began to call themselves the Éothéod. They were skilled horsebreeders and horsemen. In 2510 T.A. they responded to a plea of help from the trapped Gondorian army at the Field of Celebrant. After they helped Gondor win this important victory they were rewarded the province of Calenardhon and became known as the Rohirrim.

The Men of Dale and Esgaroth were also counted under the Northmen, as were the woodsmen of Mirkwood. Most of Rhovanion had been depopulated after the wars of the Easterlings and the Great Plague.